Optical communication technology has been rapidly improving over the years due to the development of optical fiber technologies and light sources such as semi-conductor lasers. In particular, the wavelength division multiplexing, in which optical signals having different wavelengths are transmitted through a single optical fiber, has been established as a key technology in optical communication. Further, the Erbium-doped fiber amplifier (EDFA) has been recently developed to resolve the problem of energy loss in optical signals, which is caused by long distance transmission.
In the technical field of optical communication, a wavelength band ranging from 1,530 to 1.565 nm is commonly employed. In cases where optical signals in the wavelength band are multiplexed and transmitted through a single optical fiber, each of the optical signals has a different refraction index with respect to each wavelength. The different refractive indices to the optical fibers, which depend on the wavelength, cause a dispersion effect. That is, as the transmission distance becomes farther, the optical signals transmitted through the single optical fiber spread out along the time axis. Also, as the transmission distance becomes farther, the dispersion of the optical signals becomes even more prominent to the degree that the transmitted optical signals overlap each other. Thus, it is difficult to discriminate the optical signals at the receiving end of the optical transmission system. As such, the influence of the dispersion slope increases.
A tunable dispersion compensator adopting an optical fiber grating has been mainly used to compensate for the dispersion of these optical signals. Such dispersion compensator facilitates the connection to an optical cable, provides low transmission loss, and does not produce any nonlinear phenomenon of the optical signals. Generally, an optical signal having a central wavelength λ1 comprises the central wavelength and a plurality of wavelengths within a range (i.e., λ1±δ nm) spreading from the central wavelength λ1. In such a case, it is known that the longest wavelength (i.e., λ1+δ nm) of the optical signals causes the most severe dispersion along the time axis as the transmission distance becomes longer. This is due to a slower transmission rate than other wavelengths. On the other hand, the smallest wavelength (i.e., λ1−δ nm) of the optical signals causes a lower dispersion even though the transmission distance becomes longer due to a transmission rate that is more rapid than other wavelengths. Consequently, in order to compensate for the dispersion of the longest wavelength (i.e., λ1+δ nm) of the optical signal pulses, it may be desirable to reduce the reflection path in the optical fiber grating. In order to compensate the dispersion of the shortest wavelength (i.e., λ1−δ nm), however, it may be preferable to extend the reflection path in the optical fiber grating. This is to compensate the dispersion of the optical signal pulses caused by the long distance transmission.
Generally, the methods of controlling the variable dispersion and dispersion slope of a compensator based on an optical fiber grating may be classified into two methods. According to the first method, the optical fiber grating is divided into several or dozens of sections. Further, the refractive index of the grating is changed by heating and cooling each section at a different temperature in order to adjust the dispersion value. According to the second method, an optical fiber grating is adhered to a surface of a metal plate. In said method, the metal plate is bent to change the period of the grating and the dispersion of the optical signal is adjusted due to the changed period. However, in the first method, the variation of refractive index of the grating in each section becomes discontinuous due to the repeated heating and cooling. Further, unexpected variations of refractive indices on adjacent sections may occur due to heat conduction. Thus, the performance of the tunable dispersion compensator becomes degraded.
A bending process is performed in the second method. More specifically, one end of the metal plate, to which the optical fiber grating is adhered, becomes fixed, while the other end of the metal plate is moved so that the metal plate can be bent. Therefore, the period of the optical fiber grating may vary due to the tensile force and contractile force induced by bending the metal plate. In other words, the dispersion is compensated when the periods of the optical fiber grating become longer upon the inducement of the tensile force, while the period of the optical fiber grating becomes shorter upon the inducement of the contractile force. As such, the dispersion value, which is defined as a variation of the group delay time of wavelengths of the optical signals, can be therefore adjusted by varying the periods of the optical fiber grating. However, it is difficult to linearly control the dispersion and the control is bound to a fixed range.